Summary
- Color is the perceptual quality of light.
(Color is a subjective response by the brain to light stimulating the retina.)
- Two visual regions have the same color if a difference between them cannot be perceived by the average human eye.
- The human eye can distinguish nearly ten million colors.
- Color as a visual response should not be confused with the "color" of a pigment, which is the color one would see when viewing that pigment under typical lighting conditions.
- Although they may not satisfy the definition of a color in a fashion sense; black, white, and gray each satisfy the physical and perceptual definitions of a color.
- The color of the light coming from an object has its origin in one or more of the following processes…
- emission: the object itself is a source of light with a color determined by its spectra
- reflection: certain frequencies are reflected from the object while others are not
- transmission: certain frequencies are transmitted through the object while others are not
- interference: certain frequencies are amplified by constructive interference while others are attenuated by destructive interference
- dispersion: the angular separation of a polychromatic light wave by frequency during refraction
- scattering: the preferential reradiation of certain frequencies of light striking small, dispersed particles
- There are six simple, named colors in English (and many other languages) each associated with a band of monochromatic light. In order of increasing frequency they are red, orange, yellow, green, blue, and violet.
- The range of frequencies corresponding to each band is subject to individual, cultural, and historical factors.
- Indigo is not included in this list as it is purely a historic artifact. The word is rarely used by contemporary speakers of English to describe a color.
- The sensation of purple cannot be produced using light of a single frequency, but only by combining light near the red end of the spectrum (the lower frequencies) and the violet end of the spectrum (the higher frequencies).
- At relatively low intensities…
- monochromatic light in the red, orange, and yellow bands appear brown.
- monochromatic light in the blue band is difficult to distinguish from violet.
- Humans perceive polychromatic mixtures of light as a single color,
which may or may not look like light from a monochromatic light source.
- Polychromatic light is described physically by a spectral power distribution (often just called a spectrum), which is a graph of intensity vs. wavelength (or frequency)
- Regions with different spectra that appear to be the same color are called metamers and the effect is called metamerism.
- Regions that are metamers when illuminated by one light source may not appear to be the same color when illuminated by another light source.
- The wide variety of colors visible to humans can be approximated by mixing only a small subset of colored light sources or colored pigments.
- Color mixing can be accomplished by…
- superposition (e.g., lamp overlap, filter overlap)
- rapid alternation, faster than the persistence of vision (e.g., biased LED, rotating color wheel)
- small, nearby elements (e.g., dithering, pixels, halftone dots, photo grain, pigment mixing)
- White light is a mixture of visible frequencies of electromagnetic radiation whose appearance approximates that of a blackbody radiator with its peak wavelength in the middle of the visible spectrum.
- There is no one frequency distribution that can be identified as white light. Human vision adapts to the illumination provided by the environment so that many blackbody and non-blackbody sources appear white.
- The quality of white light emitted from a blackbody radiator is a function its temperature. This quality is known as color temperature.
- Daylight at midday is often considered the standard value of white light. It produces the same response in the human eye as a blackbody radiator at 6500 K. Visual regions with the same color temperature as midday light often appear neutral white.
- A visual region with a color temperature below 6500 K emits white light that looks reddish in comparison. For cultural reasons light of this color is called warm white even though it is from a "colder" source.
- A visual region with a color temperature above 6500 K emits white light that looks bluish in comparison. For cultural reasons light of this color is called cool white even though it is from a "hotter" source.
- A visual region looks gray if the light from it is similar to white light, but has an intensity somewhat lower than its surroundings.
- Black is the relative absence of visible light.
- A visual region that emits, reflects, or transmits much less visible light than its surroundings looks black.
- The primary colors of the human visual
system are red, green,
and blue.
| Primary colors |
| black |
+ |
red |
= |
red |
| black |
+ |
green |
= |
green |
| black |
+ |
blue |
= |
blue |
- No combination of two primary colors can reproduce a third primary color.
- Combinations of the primary colors will reproduce a wider range of colors than than can be reproduced using any other three colors.
- Combinations of primary colors follow the rules of additive color mixing.
| Additive color mixing rules |
| no light |
= |
black |
| red |
+ |
green |
= |
yellow |
| green |
+ |
blue |
= |
cyan |
| blue |
+ |
red |
= |
magenta |
| red + green |
+ |
blue |
= |
white |
- Systems that work by additive color mixing include…
- photographic and movie film (prints, slides, negatives)
- television and computer displays (CRT, LED, LCD, plasma)
- The secondary colors of the human visual system are cyan, magenta, and yellow.
- A secondary color is formed by subtracting a primary color from white light.
| Secondary colors |
| white |
− |
red |
= |
cyan |
| white |
− |
green |
= |
magenta |
| white |
− |
blue |
= |
yellow |
- Every secondary color is the complementary color or opposite color of a primary color.
- Combining primary and secondary colors of light produces light that looks white.
| Complementary color mixing rules |
| red |
+ |
cyan |
= |
white |
| green |
+ |
magenta |
= |
white |
| blue |
+ |
yellow |
= |
white |
- Combinations of secondary color pigments follow the rules of subtractive color mixing.
| Subtractive color mixing rules |
| no pigment |
= |
white |
| cyan |
+ |
magenta |
= |
blue |
| magenta |
+ |
yellow |
= |
red |
| yellow |
+ |
cyan |
= |
green |
| cyan + magenta |
+ |
yellow |
= |
black |
- The black produced by mixing the three secondary colors is of low quality.
- Systems that work by subtractive color mixing include…
- three-color printing
- pigment mixing (as in custom paints)
- The "primary colors" of the painter's color wheel are red, yellow, and blue
- When combining pigments in equal quantities…
| "Painter's" color mixing rules |
| no pigment |
= |
white |
| red |
+ |
yellow |
= |
orange |
| yellow |
+ |
blue |
= |
green |
| blue |
+ |
red |
= |
purple |
| red + yellow |
+ |
blue |
= |
brown |
- Purple is similar to magenta.
- The misidentification of these colors as "primary" is a historical artifact. A greater range of colors can be reproduced using cyan, magenta, and yellow than can be reproduced using red, yellow, and blue.
- Although this is called the painter's color wheel, no serious painter would claim it possible to reproduce every desired color from these three pigments.
- Color Spaces
- All color spaces have at least three dimensions
- RGB (red, green, blue)
- Named for the dominant wavelength of the three light sources used.
- Numbers ranging from zero to some bit number maximum (255, 65535, etc.) are used to describe the relative intensity of each of the three light sources.
- black: none of the light sources are turned on
(R = G = B = 0)
- white: all light sources are turned up as bright as they can
(R = G = B = maximum value)
- CMY (cyan, magenta, yellow)
- Named for the secondary color of the three inks when viewed under white light on a white sheet of paper.
- Numbers ranging from 0% to 100% are used to describe the per cent coverage of a blank sheet of white paper by each of the three inks.
- white: no ink on a white sheet of paper
(C = M = Y = 0%)
- black: paper completely covered with each type of ink
(C = M = Y = 100%)
- HSB (hue, saturation, brightness) a.k.a. HSV (hue, saturation, value)
- The hue angle describes the most visually dominant wavelength
- 000° = red
- 060° = yellow
- 120° = green
- 180° = cyan (similar to, but not the same as the pigment; something like sky blue)
- 240° = blue
- 300° = magenta (similar to, but not the same as the pigment; something like violet or purple)
- 360° = red
- The saturation per cent describes the vibrancy.
- 000% = desaturated (white)
- 100% = saturated (as vibrant as the system will allow)
- The brightness or value per cent describes the relative intensity.
- 000% = dark (black)
- 100% = bright (as bright as the system will allow)
- HSL (hue, saturation, lightness) a.k.a. HSI (hue, saturation, intensity)
- The hue angle describes the most visually dominant wavelength
- Same hue angles as in HSB
- The saturation per cent describes the vibrancy.
- 000% = desaturated (grayscale)
- 100% = saturated (as vibrant as the system will allow)
- The lightness per cent describes the relative intensity.
- 000% = dark (black)
- 050% = medium
- 100% = light (white)
- XYZ (tristimulus)
- Some printing schemes use more than three colors of ink
- CMYK (cyan, magenta, yellow, black)
- CMYK+spot (cyan, magenta, yellow, black, special ink or coating)
- CcMmYK (cyan, light cyan, magenta, light magenta, yellow, black)
- CMYKOG a.k.a. Hexachrome (cyan, magenta, yellow, black, orange, green)
